Advancing age is associated with a reduction in skeletal muscle protein and muscle strength; a syndrome referred to as sarcopenia. Skeletal muscle protein turnover can be studied by infusion of stable isotopically labeled amino acids and mass spectrometry. The reduction in muscle strength with aging results in physical disability and frailty, a loss of independent function, and contributes to escalating health care costs. Support from the NIA and other beneficent organizations (AARP, AFAR, Andrus Foundation, Allied-Signal) has accelerated research breakthroughs, progress, and our understanding of the mechanisms of muscle protein wasting in advancing age. Several new discoveries have enhanced our appreciation of the intricacies of muscle physiology that regulate muscle protein mass and contractile function.